1. Field of the Invention
The present invention relates generally to a wireless network such as sensor network with a tree topology, and more particularly to the selection of parent nodes in the network.
2. Description of the Related Art
Various methods have been developed for configuring a wireless network in which nodes that are not directly linked communicate in a series of hops through intermediate nodes. Unlike wireline networks, wireless networks permit a node to transmit signals to all its neighboring nodes with no additional wiring cost, so the multi-hop communication routes are often selected on the assumption that each node can communicate with all its neighboring nodes.
This assumption, however, leads to increased power consumption at the nodes. Because each node may receive signals from any of its neighboring nodes, the receiving component of the node must be kept operating and ready to receive at all times, so the receiving component constantly consumes power.
In an alternative method, the nodes (wireless terminals) of the network are divided into relaying nodes and non-relaying nodes, and communication is routed only through the relaying nodes.
In a method described in Japanese Patent Application No. 2009-55301, a non-relaying wireless terminal referred to as an end device switches off the power of its transmitting and receiving components during sleep periods to reduce power consumption. Each non-relaying wireless terminal selects one neighboring relaying wireless terminal, communicates only with the selected wireless terminal, and communicates only at restricted timings, thereby reducing power consumption. The role of each wireless terminal (relaying or non-relaying) is fixed, however, and cannot be changed. This creates a problem if a new wireless terminal is added to the network in a position where the only neighboring wireless terminals are non-relaying wireless terminals.
Similar problem are apt to occur if a relaying wireless terminal drops out of the network, or if a link between a relaying wireless terminal and another wireless terminal becomes inoperable. This makes the network vulnerable to battery failure and various other types of failure that may leave some non-relaying wireless terminal unable to communicate with any relaying wireless terminal. Such a network lacks reliability.